(1) Field of the Invention
It is a further object of the present invention to provide a process for the manufacture of the copolymer solutions and for the use thereof as a binder component, wherein the sheet-like structures which can be produced therefrom have considerably improved porperties in various respects. This includes the property that the lacquer which has been applied to a metallic substrate and which contains the new copolymer solutions in combination with organic polyisocyanates, shall, after drying in the air for three to four days at temperatures of about 20.degree. C., cure in such a way that the film no longer swells under the influence of water and also cannot be removed mechanically by scratching, even if the film is exposed to water for a brief period at temperatures of 50.degree.-70.degree. C.
It is a further object of the present invention to provide an acrylic resin, containing hydroxyl groups, which, in combination with polyisocyanates, provides the binder foundation for air-drying lacquers and also stoving lacquers which are suitable for top coats of lacquer in the automobile industry, the top lacquer being present after stoving in layer thicknesses of about 40 to 80 .mu.m and the top lacquer being located directly on top of the stoved primer of electro-coating lacquer.
In the modern methods of the present time for lacquering motor vehicles in the automobile factory, the coachwork parts are usually provided, after phosphating, with a primer of electrocoating lacquer and the latter is stoved to give a plastic coating. A spraying lacquer or a coating composition, which is usually designated a filler, is then applied to this plastic coating, usually by a spray process, and is again stoved.
The pigmented formulation of automobile lacquer is then applied, in the desired pigmentation, to this two-layer coat of lacquer or primer, by spraying, and the automobile body is stoved again.
The complete coating, which, as explained above, consists of three layers, has a layer thickness of 70-100 .mu.m after stoving, of which the proportion relating to the outer layer of top lacquer is 30 to 40 .mu.m. In the course of methods of rationalisation in the automobile industry efforts are being made to change over from the three-layer system to a two-layer build-up. In this, the electrocoating lacquering or another equivalent primer is intended to form the first coating layer and the final lacquering with the pigmented lacquer is intended to be built up thereafter, the total thickness of the layers applied being intended, however, to have the same layer thickness. Using the conventional automobile top lacquers, however, it is not possible to produce, from pigmented lacquer formulations in the customary spraying and stoving treatment, coatings of a type which, after drying in the air or after stoving, give lacquer films which are completely uniform (that is to say free from so-called curtaining and free from craters and blisters).
It is the object of the present invention to provide an acrylic resin, containing hydroxyl groups, which, in combination with polyisocyanates, provides the binder foundation for air-drying lacquers and stoving lacquers which are suitable for top coats of lacquer in the automobile industry, the top lacquer being present after stoving in layer thicknesses of about 40 to 80 .mu.m and the top lacquer being located directly on top of the stoved primer of electrocoating lacquer or on top of another suitable primer.
In addition, the less viscous copolymer solutions used according to the invention exhibit a better absorption of pigment both when using inorganic pigments, such as, for example, titanium dioxide, or organic pigments, such as, for example, carbon black. When titanium dioxide is used as the pigment, the copolymers manufactured according to the invention, and the reactive lacquer manufactured therefrom, still produce high-gloss films at a pigment/binder ratio of 1.5 to 2:1, whilst the known copolymers and the known reactive lacquers manufactured therefrom exhibit a distinct falling off in gloss at the same level of pigmentation. In addition, it has been found that the copolymers manufactured according to the invention and the reactive lacquers manufactured therefrom produce high-gloss lacquers with carbon black, whilst the known copolymers and the reactive lacquers manufactured therefrom display matt films at the same level of pigmentation.
It is a further object of the invention to provide a process for the manufacture of soluble organic copolymers which are obtained as a solution with a solids content of 65 to 75% by weight, the copolymer solutions, when they have been adjusted to be ready for spraying, having a viscosity, at 25.degree. C., of 25 seconds, measured in the DIN cup with a four mm flow orifice, and a solids content of 40 to 65% by weight, preferably 45 to 65% by weight. In this way the copolymer solutions manufactured according to the invention take account of the demands of environmental protection for binders of high solids content and low solvent content.
The upstanding advantage of the copolymer solutions manufactured according to the invention is the possibility that the hydroxyl groups of the copolymers are crosslinked with polyisocyanates at room temperature and at elevated temperatures and give lacquers which have a high solids content when adjusted to the ready-to-spray state, at a viscosity of 40 seconds, measured in the DIN cup having a 4 mm flow orifice, at 25.degree. C., and a solids proportion of 61 to 80% by weight, preferably 65 to 80% by weight, in the clear lacquer.
(2) Prior Art
Numerous proposals have been disclosed to manufacture solvent-resistant and alkali-resistant lacquers by reacting polyisocyanates and copolymers containing hydroxyl groups and to convert them into coatings. DT-AS No. 1,247,006 describes a process for the production, by the polyisocyanate polyaddition process, of alkali-resistant sheet-like structures which are obtained from copolymers containing hydroxyl groups, and polyisocyanates, but which are not adequately water-resistant after a reaction time of three to four days at 20.degree. C. These coatings, obtained by this known process, are, therefore, not suitable as top lacquers for external lacquering which are resistant to weathering, since blistering takes place after a very short time and the adhesion of the lacquer to the various metallic substrates falls off and as a result, the lacquer is detached from the substrate.
It is also known that polyhydroxy compounds of polyols can be cured with polyisocyanates in order to obtain crosslinked lacquer coatings with good resistance properties. These combinations also display too low a resistance to water and a fairly low resistance to weathering of the cured lacquer films.
The particular advantage of the copolymer solutions manufactured according to the invention is that the viscosity falls when the hydroxyl group content in the copolymers is increased. On dilution with xylene to 25 seconds at 25.degree. C., measured in the DIN cup having a 4 mm flow orifice, the solids content of the copolymer solutions manufactured according to the invention then increases.
It is found, suprisingly, that the copolymers which have been manufactured according to the invention and have a hydroxyl group content of 3.5 to 6.5, preferably 4.5 to 5.5,% give lower viscosities than the known copolymers which have the same hydroxyl group content. Thus, in a 70% strength by weight solution in ethylglycol acetate, the copolymer solutions manufactured according to the invention give viscosities of U to Z.sub.4, preferably X to Z.sub.3, measured by the Gardner-Holdt method at 20.degree. C. This observation is proved by a comparison between the copolymer solution, manufactured according to the invention, from Example 1 and the known comparison copolymers 3 and 4. As 70% strength by weight solutions in ethylglycol acetate, the comparison copolymers 3 and 4 giver higher viscosities in excess of Z.sub.6, measured by the Gardner-Holt method at 20.degree. C.
When combined with polyisocyanates, the copolymers manufactured according to the invention have the following advantages, compared with the known copolymers: better resistance to solvents, higher abrasion resistance and better resistance to weathering in the Florida climate. In addition, the copolymers manufactured according to the invention, when combined with polyisocyanates and diluted with acetone, xylene or butyl acetate to a viscosity of 40 seconds at 25.degree. C., measured in a DIN cup having a 4 mm flow orifice, give lacquers of higher solids content which have a solids content of 61 to 80% by weight, preferably from 65 to 80% by weight, in the clear lacquer. When such clear lacquers or pigmented lacquers are applied to steel sheets by spray application and are exposed to the air for a brief period and subsequently stoved, high dry film layer thicknesses of 70 to 80 .mu.m are obtained, which do not exhibit crater-formation or blistering.
The known lacquer combinations which are described in French patent Specification No. 1,556,309, give only a solids content from 10 to 60% by weight in the clear lacquer, when they are diluted with acetone, butyl acetate or xylene to 40 seconds at 25.degree. C. When such clear lacquers or pigmented lacquers are applied to steel sheets by spray application and are exposed to the air for a brief period and subsequently stoved, lower dry film layer thicknesses of 40 to 50 .mu.m are obtained which, in addition, exhibit crater-formation and blistering.